applications such as compressors and jet engines. But their Achilles heel can be a thirst for water. Trevor Gauntlett looks at one chemical companythat is trying to overcome thishurdle for biodegradable esters.

Lubricant formulators have a commonly held belief that biodegradable esters are hydrolytically unstable. As biodegradability is desirable in some applications and hydrolytic instability is not desirable at all, many formulators will seek alternative components as a first choice, often returning to esters when the cost of more exotic ingredients is prohibitive.

A lubricants ability to remain stable in the presence of water and not degrade is known as its hydrolytic stability.

An ester is made by reacting a carboxylic acid with an alcohol. Hydrolysis can reverse this reaction and regenerate the carboxylic acid. This has obvious implications for corrosion, as well as the breakdown of the ester base fluid at the molecular level, as the acid liberated in the reaction can then go on to attack another ester molecule.

Not Correlated

But the correlation of biodegradability and hydrolytic stability is not absolute, according to Ronald Hoogendoorn of Patech Fine Chemicals Co. Ltd, an oleo-chemical derivative company headquartered in Taipei, Taiwan. In a presentation to the ACI European Base Oils and Lubricants Summit in Rotterdam
in November, Hoogendoorn explained the different modes of degradation of esters due to biochemical (biodegradability) and chemical (hydrolysis) attack and then showed examples of modifications to esters that could enhance their hydrolytic stability but retain biodegradability, as well as other performance characteristics.

Model Studies

Patech closely analyzed a number of esters selected from their commercial range, plus a few one-off laboratory samples that had similar structures but in some cases widely different properties. Esters with roughly the same molecular weight and shape, and therefore, roughly the same volatility, viscosity and viscosity index, can have quite different biodegradability and hydrolytic stabilities.

The companys scientists confirmed that one of the keys to enhancing hydrolytic stability in esters is to ensure that the alpha position to the carbonyl group of the ester (as circled in Figure 1 below) is sterically hindered by adding bulky groups to the molecule to slow down certain chemical reactions. This is best achieved in small molecules by creating the ester from an acid that has branching at the alpha position. Doing this with an alkyl group – a chain of carbon and hydrogen atoms – not only provides some steric bulk around the alpha carbon, but it also reduces the acidity of the remaining hydrogen atom attached to that carbon. More alpha substituents can increase hydrolytic stability effectively.

In the example below, illustrated in Figure 1, Patech compared the two esters that can be made from hexadecanoic (palmitic) and 2-ethylhexyl precursors. A higher acid number during the test indicates a greater degree of degradation. Hexadecyl 2-ethylhexanoate – the red line – contains a branch at the alpha position to the carbonyl group. The acid value of this ester undergoing hydrolysis by a modified ASTM D2619 method (test temperature raised from 90 degrees Celsius to 120 C and an oil-to-water ratio of 4:1) increases at half the rate compared with the ester with no branching – the 2-Ethylhexyl hexadecanoate represented by the blue line.

Biodegradation

There are two principal biodegradation pathways for esters, according to Hoogendoorn. One is through hydrolysis of the ester – chemical breakdown by water – then oxidation of the alkyl chain beginning at the beta position, relative to the carbonyl group. However, if hydrolysis is hindered, then other degradation processes can become important. Microbial attack is one of these and can involve oxidation at the omega sites relative to the acid group. So it is possible to build esters that enhance these biodegradation routes.

The esters in Figure 1 have broadly similar biodegradation behavior of around 80 to 90 percent in 28 days. (This data is taken from the registration dossiers 15089 and 11691 filed with the European Unions Restriction, Evaluation and Authorization of Chemicals system.)

Enhancing hydrolytic stability and biodegradability is a trade-off, said Hoogendoorn. The mechanisms are similar but are not the same. He believes that Patech can optimize the different influences of branching on the balance between hydrolytic stability and biodegradability and find suitable structures to meet different application requirements.

New Products

Patech has used this knowledge when producing its new biodegradable ester range from ISO 46 to ISO 1000. This includes variants with enhanced hydrolytic stability, due to modified structures, by application of the model studies described here.

Its always good to see underlying science being used to inform product innovation, Steve Boyde, a consultant on esters and synthetic lubricants base fluids at bm4tech Ltd, a consulting company from the United Kingdom, told LubesnGreases. I hope that Patech will be able to deploy this knowhow creatively to offer higher-performing ester base fluids to the market.

Boyde, who authored the esters chapter for the forthcoming third edition of Synthetics, Mineral Oils, and Bio-Based Lubricants, said he expects truly novel esters to be much more expensive, as the basic molecules that are the raw materials for esters with novel properties are. The question is then whether the market would be willing to pay for them.

Ester chemistry is a bit like building structures with Lego. It lets you snap together lots of interesting and useful structures, but you can only work [commercially] with the rather limited range of building blocks that are available and affordable.

For example, if you want to exploit acid alpha branching to control hydrolysis, then the only currently available, cost-effective raw material with that structure is 2-ethylhexanoic acid.

Having to build the structure around a C₈ monoacid puts constraints on what you can do with other relevant properties, like polarity or viscosity index, said Boyde.

Even if such raw materials were available, the current source for many of them is petrochemicals, which could be a disincentive to some customers. While it is not a requirement in most cases for biodegradable lubricants to also be bio-sourced, there are significant segments of markets for biodegradable lubricants where the customer would prefer the fluids to be bio-sourced lubricants.

Developments elsewhere could also influence the market. According to Boyde, The stability in use versus biodegradability trade-off is real, but perhaps less of an issue now than it was when environmentally acceptable lubricants were first introduced. Original equipment manufacturers and operators have got better at keeping the water out.

This means that the operational conditions that lead to hydrolysis (usually the presence of both water and acid) are less prevalent. In addition, ester producers have tightened product specifications by, for example, reducing the acid number of their products, while formulators have learned which additives to avoid. Therefore, the same ester structure now lasts longer than it used to.

Performance at the Right Price

Much of the discussion surrounding biolubricants at the summit centered on the fact that technical performance is much higher in the hierarchy of considerations than biodegradation. However, cost is also very high in that hierarchy. This begs the familiar question for Patech Fine Chemicals, and other ester manufacturers: Can they enhance performance of an ester in a critical area at a cost that the market is willing to pay? If so, their fundamental studies will have been worthwhile.